Such a transmission is in particular usable in motor, rail or other vehicles, but also to drive any kind of variable speed machines, for example winches, ventilating fans, compressors, farmiing or public works machines, crushers, weaving machines, as well as to drive constant speed machines such as generators using wind power engines or variable speed turbines.
To drive motor vehicles, a standard type automatic transmission comprises a switchable gearbox, preceded by a fluffy torque converter. The latter presents losses of energy. At the present time, we are seeking to reduce these losses by adding a high gear clutch, as well as a power by-pass via a positive transmission channel arranged parallel to the converter. To make such a transmission work automatically and perfectly, an electronic control device is necessary. All these means further increase the complexity, the weight and the construction and maintenance costs.
This is why we are seeking to replace these transmissions by mechanisms of other types, which are continuously variable over the whole range of speeds of the output shaft, to avoid using a switchable gearbox. One known solution uses a variable speed belt and pulley drive with a variable diameter. These variable speed drives can have an automatic setting system based on the centrifugal force. However, the belt transmits the loads by friction and that always implies losses of energy and overheating problems which limit the application of these transmissions to the field of lower power outputs. Furthermore, there is always a risk of the belt slipping.
Moreover, we know of mechanical couplings allowing a difference in speed between the input shaft and the output shaft, due to a centrifugal torque compensation device which acts on a differential gear linking the input shaft to the output shaft. The patent GB-A-995305 describes a coupling of this type, where the planet wheels of the planetary differential gear each carry a secondary planetary gear train formed by a secondary planet wheel carrier integral with the main planet wheel four secondary planet wheels each provided with an eccentric weight, and a secondary planet pinion designed to hold each weight in a constant position in relation to the direction of the centrifugal force it is subject to, so that this force creates a centrifugal torque which is as even as possible while the unit rotates. A manual control device can act on the position of each secondary planet wheel to select one of four positions of the weights, i.e. in the respective positions of forward drive, neutral, reverse drive and lock against overdrive. In an alternative, the secondary planet wheels are replaced by a set of articulated levers carrying mobile weights which create a centrifugal torque on the main planet wheel. This is a complicated and delicate construction which is not suitable for transmitting high torques at high speeds. Above all, the major drawback inherent in such couplings is that they can not operate as torque converters, because there is no point of support or reaction point on a fixed part, for example a case. This is why they can not apply a torque greater than the input torque to the output shaft, whatever the speed of the output shaft.
The patent FR-A-842 760 describes a transmission of the kind mentioned in the preamble to claim 1. This device known under the name of Hobbs transmission actually operates like a torque converter, due to a bearing pressure on the case which makes it possible to multiply the torque when the output speed is low. The device comprises eccentric weights on planet wheels of two planetary gear trains which are centered on the common axis of the input and output shafts. During one revolution of a planetary gear train, the centrifugal forces produce varying centrifugal torques, i.e. which change direction because of the rotation of the planet wheels. This results in varying pulses, of which those in a positive direction are selected by a freewheel device which transmits them to the output shaft via an elastic or inertia device, which smooths the pulsed output torque. As freewheel devices do not at all withstand such varying or pulsed torques at a high frequency, which is of the same order of magnitude as the speeds of the input and output shafts, transmissions of this kind have not been able to break through in actual pratice.
The object of the present invention is to create an automatic continuously variable transmission which makes it possible to avoid the above-mentioned drawbacks of prior transmissions. The invention relates in particular to a continuous transmission entirely positive, avoiding any transmission of energy by friction or by a flow of fluid, which is capable of transmitting high and continuous torques in a wide output shaft speed range, particularly for the applications for driving vehicles, or conversely in a wide input shaft speed range if the output shaft has to rotate at a constant speed.